1. Field of the Invention
The present invention relates to a method of calibrating LED writers and more particularly to a method for correcting for non-uniformity errors associated with such writers and particularly to the correction of errors arising from incorrect spacing of LEDs relative to each other on a printhead.
2. Description Relative to the Prior Art
With reference to PCT publication No. WO 91/10311, the pertinent contents of which arc incorporated herein by reference, in an LED (light-emitting diode) based grey scale electrophotographic printing system, exposure is usually controlled by the length of time each LED is turned on with the emitted light intensity held constant. The LED on-time varies imagewise and the resulting exposure is given by the product of this time and the intensity. The printhead itself is comprised of several thousand individual LEDs and, because of variation in LED response, the light intensity varies from LED to LED within the same printhead for the same amount of energizing current. This results in a non-uniform exposure of an otherwise uniform field.
For each level of gray the number of exposure times that can be requested is potentially equal to the number of LEDs in the printhead. Thus, for a j-bit gray level system that uses an LED printhead with N LEDs as the writer, the number of possible exposure times that can be requested is N (2.sup.j -1), excluding the null exposure level (white). For N=4000 (typical) and j=4, this number is 60,000. There is no economical printhead architecture that is capable of generating nearly as many exposure times as this. The number of exposure times that a typical LED printhead can generate is determined by its controller circuit. For a printhead with a k-bit controller (say for example k=6), the total number of exposure times that can be generated is 2.sup.k, with j&lt;k but significantly less than N (2.sup.j -1). It is the goal of a non-uniformity correction algorithm to use the total number of exposures that can be requested to generate an optimum look-up table (LUT).
In a preferred look-up table approach to providing nonuniformity correction described in U.S. Pat. No. 5,255,013, the LUT is realized by two look-up table memories. A first LUT memory represents the grading of the approximately 4,000 or more LEDs into 256 categories according to their respective brightness levels. As is known, even though the same current be driven through each LED, LEDs respond differently and their light outputs vary. Some may be brighter than others with the very brightest being assigned category 255 and the least brightest being assigned category 0.
The first LUT memory provides reasonable approximation for correction. In theory the desired grey level of exposure C.sub.j, for grey level "j" should be in accordance with the following equation B.sub.i .times.T.sub.ij =C.sub.j where B.sub.i is the brightness of a particular LED "i" and T.sub.ij is the required exposure time for an LED of brightness B.sub.i to form an exposure C.sub.j. The grey levels C.sub.j may be noted empirically by sensing the brightness of each LED and identifying the time period T.sub.ij required to record each of the various grey levels. The LEDs can then be grouped into appropriate categories or bins of brightness and a second look-up table memory created to approximate the print duration by assigning say a 6-bit corrected printing data signal for each grey level for each of the particular categories of brightness to which LEDs have been assigned.
While this approach is generally satisfactory, a problem still exists with regard to nonuniformity errors from LEDs at the ends of arrays. Typically, in the manufacture of printheads, the about 4000 or more LEDs are fabricated in arrays of say 64, 128 or 196 LEDs to an array. Arrays are then assembled end to end upon a printhead to make up the number of LEDs required. While LEDs are uniformly spaced or pitched on each array because of the accuracy of the photolithographic fabrication process, the spacing between LEDs at the ends of butting arrays do not generally possess this uniformity of pitch. Heretofore, the approach of the prior art is to adjust spacing as best as possible and/or otherwise make do with the error that improper spacing imposes.
It is, therefore, an object of the invention to improve upon the exposure non-uniformities created by improperly spaced end LEDs which thereby has the advantage of either increasing uniformity and/or permitting looser tolerance in butt spacing of end LEDs on arrays.